Projection televisions are well known. In general, a projection television set or projector includes three cathode ray tubes (CRTs), corresponding to the primary colors, red, blue, and green. Associated with each of the CRTs is a projection lens assembly, which is comprised of a plurality of optical lens units. Overall, a CRT has an attached faceplate, and the function of the lens assembly is to magnify the image appearing on the faceplate of the CRT, and thereby project it onto a viewing screen, which is much larger than the faceplate of the CRT. For a typical layout of the optical lens units in a projection television lens assembly see U.S. Pat. No. 4,776,681.
Basically a projection television lens assembly is made up of a plastic tubular member, generally referred to as a lens cell. It is the lens cell that has a plurality of optical lens units mounted within it. The lens cell itself is mounted within another plastic tubular member referred to as a focus mount. The mounting of the lens cell within the focus mount allows axial movement of one with respect to the other, which results in focusing the image onto the screen. An apparatus for achieving such adjustment is shown in U.S. Pat. No. 5,276,555. After the image is focused, the axial position of the lens cell is then locked with respect to the focus mount. In the past, the basic approach has been to screw in a bolt through a slot on the focus mount, and into a boss that is integral with the lens cell, which was then followed by a washer and a nut over the bolt. By turning the nut clockwise, that is, tightening the nut, the lens cell and the focus mount were pulled together to create a frictional lock. However, when the nut was tightened securely to maintain the focus of the projection television lens assembly, this led to the deformation of the plastic lens cell and the focus mount. The tightening of the nut also caused the lens cell containing the optical lens units within the focus mount to tilt, thus, causing an increase in flare on the image appearing on the screen.
An object of the invention is to provide a focus position lock device that maintains a focus position in a projection television lens system, and prevents any tilting of the lens cell containing the optical lens units. Another object of the invention is to provide a focus position lock device that prevents any distortion of the lens cell and the focus mount in a projection television lens assembly.